Plastic integrated circuit device package having exposed lead surface

ABSTRACT

Methods for forming packages for housing an integrated circuit device are disclosed. In one embodiment, a plastic sheet having an first surface is provided. An array of package sites is formed on the first surface of the plastic sheet. The package sites each include a metal die pad and leads surrounding the die pad. The package sites may be formed by applying a first metal layer on the first surface of the plastic sheet, and then applying a second metal layer in a pattern that defines the die pads and leads of the package sites. The first metal layer is then selectively etched using the second metal layer as an etch mask. Next, an integrated circuit die is placed on each die pad of the array. The die is electrically connected to the leads surrounding the respective die pad. An encapsulant is applied onto the first surface of the plastic sheets so as to cover the package sites. The plastic layer is removed by peeling, by dissolving the plastic sheet, or dissolving an adhesive connection between the plastic sheet and the encapsulated array. The packages are then singulated from the array.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation-in-part of U.S. patentapplication Ser. No. 09/383,022 (attorney docket M-5311 US), which isentitled “Method of Forming an Integrated Circuit Package Using PlasticTape as a Base” and was filed on Aug. 25, 1999.

FIELD OF THE INVENTION

[0002] The present invention concerns a method of forming plasticpackages for integrated circuit devices.

BACKGROUND OF THE INVENTION

[0003] A problem with conventional plastic packages is that theirinternal leadframes limit reduction of the size of the packages.Practitioners have attempted to reduce the size of packages byeliminating internal leadframes, as is shown in U.S. Pat. No. 4,530,142to Roche et al.

[0004] Roche et al. begins with a metal temporary substrate. A layer ofa low melting-point alloy is applied onto to the metal temporarysubstrate. Next a plurality of metal die pads and leads are formed onthe low-melting point alloy layer. An integrated circuit device isplaced on each of the die pads and connected to the leads surroundingthe respective die pad. The integrated circuit devices are thenencapsulated in a single block of encapsulant material. Individualpackages are then cut from the block of hardened encapsulant.

[0005] The methods and package of Roche et al. have foreseeabledisadvantages. For example, the use of the metal temporary substrate andlow-melting point alloy layer increase costs and manufacturingdifficulty. Further, the packages are believed to be unreliable becausethe contacts could easily be pulled from the encapsulant material.

[0006] A package marketed by Toshiba Corporation of Japan under the name“BCC” is believed to be made as follows. A copper sheet is partiallyetched through in certain locations, forming pockets isolated byunetched copper. A central pocket is surrounded by several smallersatellite pockets. The copper sheet is then masked, leaving the pocketsexposed. Next, the pockets are plated with layers of gold, nickel, andgold. An integrated circuit device is placed in the central pocket. (Insome embodiments, there is no central pocket, so the device is simplyplaced on the copper sheet.) Bond wires are connected between the deviceand the satellite pockets. Next, the device and bond wires areencapsulated. Finally, the remainder of the copper sheet is etched awayby acid, forming a completed package. Once the copper is removed, themetal plated into the satellite pockets forms the leads of the package,and the metal plated into the central depression (if any) is the diepad.

[0007] This process is believed to have several disadvantages. First,the use of acid to dissolve the remainder of the copper plate afterencapsulation creates a significant possibility of contamination, sincesuch acids are generally regarded as dirty. Second, the package issubject to failure, because the leads are attached to the package onlyby the bond wire and by the adhesiveness of the encapsulant to the innersurface of the plated pocket. Thus, the leads could easily be detachedfrom the bond wire and package body. Third, epoxy encapsulant materialsometimes does not adhere well to gold.

[0008] Accordingly, there is a need for a small and reliable packagethat is easier and less expensive to manufacture than prior artpackages.

SUMMARY OF THE INVENTION

[0009] The present invention includes a method of manufacturing apackage for housing an integrated circuit device. In one exemplaryembodiment, Step 1 provides a plastic sheet having an adhesive firstsurface. The plastic sheet may be plastic tape. Step 2 attaches a metalsheet onto the first surface of the plastic sheet, and then forms anarray of package sites by selectively removing portions of the metalsheet. Each package site includes a die pad and a plurality of satelliteleads around the die pad. Step 3 attaches an integrated circuit deviceto each of the die pads. Step 4 connects a conductor, such a bond wire,between each of a plurality of conductive pads on the integrated circuitdevice and one of the leads of the respective package site. Step 5applies an encapsulating material onto the first surface of the plasticsheets, integrated circuit devices, the leads, and the electricalconductors of each package site. Step 6 hardens the encapsulatingmaterial. Step 7 removes the plastic sheet. Optional Step 8 appliessolder balls to the exposed surfaces of the leads of the package sites.Finally, Step 9 separates individual packages from the encapsulatedarray. An alternative embodiment in an LCC style package requires nosolder balls.

[0010] An embodiment encompassed within the present invention includesforming a reentrant portion (or reentrant portions) and aspirates on theside surfaces of the die pads and leads of the package sites. During theencapsulation step, the encapsulant material flows into the reentrantportions and aspirates. The reentrant portions and aspirates engage theencapsulant material and lock the die pad and leads to the encapsulantmaterial of the package.

[0011] The present invention overcomes the disadvantages of the priorart by, among other things, the use of an inexpensive plastic sheet as abase for forming the packages, and by the formation of encapsulantjacking features on the side surfaces of the die pad and leads. Theseand other advantages will become clear through the following detaileddescription.

BRIEF DESCRIPTION OF DRAWINGS

[0012]FIG. 1 is a flow chart of a method of forming a package for anintegrated circuit device.

[0013]FIG. 2 is cross-sectional side view of a plated metal sheet on aplastic sheet.

[0014]FIG. 3 is a cross-sectional side view of an array of package siteseach including a die pad and leads.

[0015]FIG. 4 is a top plan view of the array of package sites shown inFIG. 3.

[0016]FIG. 5 is a cross-sectional side view of these integrated circuitdevices placed on and wire-bonded to leads of respective package sites.

[0017]FIG. 6 is a cross-sectional side view of an array of package sitesafter encapsulation by either liquid encapsulation or moldingtechniques.

[0018]FIG. 7 is a cross-sectional side view of the array of FIG. 5 afterremoval of the plastic sheet and attachment of solder balls to theleads.

[0019]FIG. 8 is a cross-sectional side view an inverted array ofpackages after the encapsulant material is cut with a saw.

[0020]FIG. 9 is a cross-sectional side view of a completed package.

[0021]FIG. 10 is a top plan view of an array of package sites with abead of adhesive material around the array.

[0022]FIG. 11 is a cross-sectional side view of a completed packageshowing a first alternative side surface of a die pad and leads.

[0023]FIG. 12 is a cross-sectional side view of an array of individuallymolded package bodies.

[0024]FIG. 13 is a cross-sectional side view of a completed packageshowing a second alternative side surface of a die pad and leads.

[0025]FIG. 14 is a plan view of a package site having two rows ofstaggered leads around the die pad.

[0026]FIG. 15 is a flow chart of an alternative method of making apackage.

[0027]FIG. 16 is a cross-sectional side view of a plastic sheet having afirst metal layer and a second metal layer thereon that serves as a maskfor etching the first metal layer.

[0028]FIG. 17 is a cross-sectional side view of the die pad and leads ofa package site formed by the method of FIG. 14.

[0029]FIG. 18 is a flow chart of an alternative method of forming apackage.

[0030]FIG. 19 is a cross-sectional side view of a plastic sheet with twolayers of an electrically-conductive media thereon, where the layersdefine an array of package sites.

DETAILED DESCRIPTION

[0031]FIG. 1 is a flow chart of an exemplary method 5 in accordance withthe present invention for forming a package for an integrated circuitdevice. An example of a completed package 37 that may be formed bymethod 5 of FIG. 1 is shown in FIG. 9.

[0032] As will become clear from the discussion the assembly method ofFIG. 1, package 37 of FIG. 9 includes a semiconductor integrated circuitdevice 28 mounted on a metal die pad 20. A plurality of metal leads 24are adjacent to die pad 20 and are connected by bond wires 29 toconductive pads (not shown) on device 28. Encapsulant material 32 formsthe package body, and covers device 28, bond wires 29, and the upper andside surfaces of die pad 20 and leads 24. The lower surface of die pad20 is exposed. Solder balls 34 are attached to the lower surface ofleads 24, although solder balls are optional.

[0033] Again, FIG. 1 provides a method 5 of making a package likepackage 37 of FIG. 9. Referring to FIGS. 1 and 2, Step 1 of FIG. 1provides a plastic sheet formed of a plastic material. Plastic sheet 10has a first surface 11 and an opposite second surface 12. An adhesive ispresent on or applied to first surface 11 of plastic sheet 10.

[0034] Plastic sheet 10 may be a segment having a size of, for example,5 cm by 20 cm, and a thickness of about 25 to 75 microns. The size andthickness of plastic sheet 10 can vary. Alternatively, plastic sheet 10may be a plastic tape that is on a roll. As described below, the processof FIG. 1 may be performed as a reel to reel process.

[0035] Plastic sheet 10 is formed of a conventional plastic materialsuch as polyvinyl chloride, polyvinyl alcohol acetate, polypropyleneacetate, vinyl, polyethylene, mylar, or polyimide. An example brand ofplastic tape is KAPTON® polyimide tape from the Dupont Company.

[0036] Step 2 of FIG. 1 provides a patterned metal sheet on the firstsurface of the plastic sheet. FIGS. 2 and 3 show one embodiment of amethod of performing Step 2 of FIG. 1.

[0037] Referring to FIG. 2, a metal sheet 13 is first attached to theadhesive first surface 11 of plastic sheet 10. Metal sheet 13 of FIG. 2has a first surface 14 attached to first surface 11 of plastic sheet 10,and an opposite second surface 15. In an alternative embodiment, anadhesive material is applied to first surface 14 of metal sheet 13rather than to first surface 11 of plastic sheet 10.

[0038] In FIG. 2, metal sheet 13 consists of an underlying first metallayer 16 on first surface 11 of plastic sheet 10, and a second metallayer 17 that is plated onto a top surface of first metal layer 16opposite first surface 11 of plastic sheet 10. First metal layer 16 ofmetal sheet 13 may be formed, for example, of copper, a copper alloy, orAlloy 42. Plated metal layer 17 may be nickel gold, silver, platinum,palladium, or another noble metal. Using nickel/gold plating or silverplating may facilitate bond wire connections. Where copper is used formetal sheet 13, without plating, copper bond wires may be used. Althoughnot shown in FIG. 2, first surface 14 of metal sheet 13 also may beplated. Metal sheet 13 may have a thickness of, for example, 100 to 250microns.

[0039] Next, metal sheet 13 is etched to form an array of package sites.For example, such etching is performed by chemical etching using apatterned photoresist mask. In such a process, a layer of photoresist isapplied onto second surface 15 of metal sheet 13. Next, the photoresistis exposed to light through a patterned reticle or the like, and issubsequently developed to form a mask. Chemicals are sprayed orotherwise applied to the masked metal strip, and exposed portions ofmetal are etched away, leaving the desired pattern. Typically, acids areused to etch the metal. Obviously, plastic film 10 must not besubstantially attacked by the chemical used to etch metal sheet 13. Acleaning step may be necessary to remove residue or other undesirablematerials after Step 2 (or any of the other steps) of FIG. 1.

[0040]FIG. 3 is a cross-sectional side view of an array 18 of threeidentical package sites 19 formed by the above-described chemicaletching of metal sheet 13. FIG. 4 is a top plan view of array 18 of FIG.3.

[0041] Referring to FIGS. 3 and 4, each package site 19 includes a metaldie pad 20 and metal leads 24. Die pad 20 and leads 24 are formed frommetal layer 16 and metal layer 17 of metal sheet 13 of FIG. 2. FIG. 4shows that each die pad 20 has a rectangular perimeter and is surroundedon each of its four sides by four leads 24. Leads 24 also have arectangular perimeter.

[0042] The perimeter shapes of die pad 20 and leads 24 can vary. Forexample, leads 24 can have a rectangular or circular perimeter. Also,the number and positioning of the leads can vary. For example, 64 or 128leads may be selected. As another example, instead of having single rowsof leads 24 around die pad 20, two or more staggered rows of leads 24may be provided adjacent to two or all four sides of die pad 20. FIG. 14is a plan view of an exemplary embodiment of a package site 19 where twostaggered rows of leads 24 surround die pad 20.

[0043] In FIG. 3, each die pad 20 has a first surface 21 attached tofirst surface 11 of plastic sheet 10, an opposite second surface 22, andside surfaces 23 at the periphery of die pad 20 between first surface 21and second surface 22. Each lead 24 has a first surface 25 attached tofirst surface 11 of plastic sheet 10, an opposite second surface 26, andside surfaces 27 at the periphery of lead 24 between first surface 25and second surface 26.

[0044] Step 3 of FIG. 1 places an integrated circuit device onto each ofthe die pads. Step 4 of FIG. 1 connects a conductor, such a bond wire,between each of a plurality of conductive pads on each of the integratedcircuit device and one of the leads of that integrated circuit device'srespective package site.

[0045]FIG. 5 is a cross-sectional side view of three integrated circuitdevices 28 placed on second surface 22 of these die pads 20. Eachintegrated circuit device 28 is placed on a die pad 20 and is adhesivelyattached to second surface 22 of die pad 20 using conventional dieattach equipment and adhesives.

[0046] Each integrated circuit device 28 of FIG. 5 includes conductivepads (not shown) that are connected to internal circuitry of theintegrated circuit device. Each conductive pad is connected by aconventional metal bond wire 29 to a second surface 26 of a lead 24.Conventional bond wire equipment is used.

[0047] Step 5 of FIG. 1 applies an insulative and adhesive encapsulatingmaterial onto the array of the package sites in order to encapsulateeach of the integrated circuit devices, the leads, and the electricalconductors of each package site. Step 6 of FIG. 1 hardens theencapsulating material forming the package bodies and the exteriorsurfaces of the packages.

[0048]FIG. 6 is a cross-sectional side view of three package sites 19 ofan encapsulated array 18 after Steps 5 and 6. The three package sites 19are covered in a single block of encapsulant material 32. In particular,first surface 11 of sheet 10 and each integrated circuit device 28, diepad 20, leads 24, and bond wires 29 are covered by encapsulatingmaterial 32. Second surface 22 and side surfaces 23 of die pad 20, andsecond surface 26 and side surfaces 27 of leads 24 are covered byencapsulant material 32. Encapsulant material 32 does not, however,cover second surface 12 of plastic sheet 10, and plastic sheet 10prevents encapsulant material from covering first surface 21 of die pad20 and first surface 26 of leads 24.

[0049] Although in this embodiment three packages sites 19 areencapsulated in a single block of encapsulating material 32, the packagesites may be encapsulated individually in an alternative embodiment ofthe process. In such an alternative embodiment, individual package sitesmay be encapsulated by injection or transfer molding, among otherpossible methods.

[0050] Steps 5 and 6 of FIG. 1 may be performed in several alternativeways. Peripheral side surface 30 of encapsulated array 18 is orthogonal,to illustrate a liquid encapsulation process. Peripheral side surface 31is sloped to illustrate a molding process for encapsulating array 18.

[0051] For example, Step 5 may be performed using a liquid encapsulant.In such a method, a first step applies a contiguous bead of aconventional hardenable viscous adhesive material onto first surface 11of plastic sheet 10 around one of more package sites 19. An example beadmaterial is HYSOL 4451 epoxy from the Dexter-Hysol Company of City ofIndustry, California. FIG. 10 is a top plan view of an array 18 of threepackage sites 19 with a bead 33 of adhesive material around the array18. Bead 33 is cross-hatched in FIG. 10. Bead 33 encloses each packagesite 19. Bead 33 forms a cavity with first surface 11 of plastic sheet10 in which the three package sites 19 and integrated circuit devices 28are enclosed. Next, bead 33 is solidified, such as by heating at 150° C.for one hour. Next, a conventional, hardenable, adhesive, and insulativeliquid encapsulating material suitable for encapsulating packages ispoured within bead 33 and fills the cavity formed by bead 33 so thatpackage sites 19, integrated circuit devices 28, leads 24, die pads 20,and bond wires 29 are covered with encapsulant material. An exampleliquid encapsulant material is HYSOL 4450 encapsulant. As a final step,the encapsulant material is hardened, such as by heating at 150° C. forone hour. This embodiment of Steps 5 and 6 forms a single solid block ofencapsulant material 32 above and on array 18.

[0052] Alternatively, Steps 5 and 6 of FIG. 1 may be accomplished usingconventional plastic molding techniques and materials, such as injectionmolding or transfer molding. In such a method, a first step places array18 in a conventional two-pocket mold. The lower pocket of the mold isblanked out by a bar so that encapsulant material does not enter thelower pocket. Next, insulative encapsulant material, i.e., moldingcompound, is provided to the upper pocket of the mold. Encapsulantmaterial 32 is molded onto package sites 19 of array 18 above and onfirst surface 11 of plastic sheet 10. Integrated circuit devices 28,leads 24, die pads 20, and bond wires 29 are covered with encapsulantmaterial 32. Next, encapsulant material 32 is hardened in a conventionalmanner. Where injection molding techniques are used, exampleencapsulation materials include styrene, liquid crystal polymer, ornylon 66. Where transfer molding techniques are used, SUMITOMO 8100molding material from the Sumitomo Company of Japan or Plaskon SMT B1RCmolding material may be used.

[0053] Step 7 of FIG. 1 removes plastic sheet 10 from encapsulated array18. An example method of performing Step 7 is to dissolve plastic sheet10 in a solvent, such as acetone. The material of plastic sheet 10should be chosen in view of its ability to be dissolved, and the solventmust be compatible with the encapsulant material 32. Another method ofperforming Step 7 is simply to use a solvent to dissolve the adhesivethat was used to attach plastic sheet 10 to metal sheet 13 withoutdissolving plastic sheet 10. Plastic sheet 10 may then fall away fromthe encapsulated array or may be peeled away. Still another method ofperforming step 7, where plastic sheet 10 is polyimide, for example, isto soak array 18 in heated water (e.g., 80° C. water) for about an hour,then apply ultra violent light to sheet 10 until sheet 10 falls off ormay easily be peeled off array 18. For example, ultraviolet light may beapplied for about one minute.

[0054] Another method of performing Step 7 is to heat plastic sheet 10and then peel plastic sheet 10 from array 18. For example, the heatingcould be to a temperature of 80° C. If Step 5 is done by molding, thenplastic sheet 10 could be removed while array 18 is in the mold or afterarray 18 is removed from the mold.

[0055] Step 8 is an optional step that applies conventional solder ballsto the exposed surfaces of the leads of encapsulated package sites 19.FIG. 7 is a cross-sectional side view of array 18 of packages sites 19after removal of plastic sheet 10 and attachment of solder balls 34 tothe exposed first surfaces 25 of leads 24. Solder balls 34 are used forconnection of the package to external circuitry.

[0056] Alternatively, Step 8 may be omitted. In such a case, firstsurface 25 of leads 24 serve as connectors to external circuitry, as ina leadless chip carrier package style.

[0057] Finally, Step 9 of FIG. 1 separates individual packages from theencapsulated array. Step 9 may be performed, for example, by cuttingencapsulated array with a saw. Disposable material, such as bead 33 iscut away.

[0058]FIG. 8 is a cross-sectional side view of array 18 of FIG. 7 afterencapsulant material 32 is cut with a saw. Encapsulated array 18 isinverted and orthogonal cuts 35 in encapsulant material 32 are made.Prior to cutting, an adhesive plastic film 36 is applied to the topsurface of encapsulated array 18 to immobilize the packages during thecutting step. Such films are used, for example, in conventional wafercutting processes. Adhesive plastic film 36 is placed on the top surfaceof encapsulated array 18 before Step 7, Step 8, or Step 9. The cuttingstep segments encapsulant material 32 without fully severing plasticfilm 36. The cuts segment the encapsulant material without fullysevering the tape. When adhesive film 36 is removed from the packagetops, the formation of individual packages 37 is completed. FIG. 9 showsa completed package 37 having orthogonal side surfaces 38 at theperiphery of package 37.

[0059] Another possible technique for singulating packages 37 (FIG. 9)from the encapsulated array 18 of package sites 19 (FIG. 7) does notrequire the use of a plastic film 36 (FIG. 8). In this alternativemethod, the encapsulated array 18 is placed on a vacuum chuck formed,for example, of fritted metal. The chuck has crisscrossing grooves inthe X and Y directions. The grooves are, for example, 0.05 to 0.08 mm indepth. The grooves correspond to where cuts are to be made betweenpackage sites 19 of the encapsulated array 18. A saw cuts along thegrooves without contacting the chuck to singulate the packages. A waterspray may be used for cooling and/or removal of waste. The packages arethen removed from the chuck.

[0060] As mentioned above, plastic sheet 10 may be a continuous tape ona reel. Where plastic sheet 10 is a continuous tape on a reel, then theprocess of FIG. 1 can be a reel to reel process. For example, Step 1 ofFIG. 1 may include unrolling a plastic tape from a first reel, attachinga metal tape to an adhesive surface of the unrolled plastic tape, andthen rolling up the joined plastic and metal tapes on a second reel.Step 2 may include unrolling the second reel of joined plastic and metaltapes, patterning a selected length of the metal to form package sites19, and then rolling the etched length of package sites 19 onto a thirdreel. The second reel is advanced until the entire length of the secondreel is etched to form a third reel of package sites 19. The third reelof package sites 19 is then unrolled, and segments of selected length,for example, from four to 200 package sites 19, are subjected to dieattach, bond wire attach, and encapsulation (i.e., Steps 36 of FIG. 1).After Step 6 (encapsulant hardening), the selected length ofencapsulated package sites 18 can be cut from the third reel, andprocessed as an individual encapsulated array 18 through the remainingsteps of FIG. 1.

[0061] Alternatively, if, for example, Step 5 of FIG. 1 is performed bymolding, then a mold having multiple two-pocket cavities can be usedwhich molds individual package housings on a selected number of packagesites 19. The lower pocket is blocked with a bar so encapsulant materialonly is applied above first surface 11 of plastic sheet 10. This isshown in FIG. 12, where three package sites 19 are molded intoindividual package bodies 39. Molded webbing 40 is between theindividual package. In such a process, after Step 6, the reel of moldedpackage sites is advanced onto a fourth reel, and a second segment ofpackages is molded. The third reel of package sites is then advanceduntil all of the package sites are subjected to die attach, wire bond,and molding. Next, the fourth roll of molded packages is unrolled, andSteps 7-9 of FIG. 1 are performed as discussed above.

[0062] In package 37 of FIG. 9, side surfaces 23 of die pad 20 and sidesurfaces 27 of leads 24 are shown as having an orthogonal orientation.Alternatively, side surfaces 23 and 27 are formed with a reentrantportion for enhancing the connection between encapsulant material 32 anddie pad 20 and leads 24. Such reentrant surfaces may be formed, forexample, by a controlled chemical etch process, or by a standardchemical etch process followed by coining of the patterned metal sheet.

[0063]FIG. 11 shows an enlarged cross-sectional side view of a completedpackage 50 having alternative side surfaces 23 of a die pad 20 and sidesurfaces 27 of leads. The reentrant portions of side surfaces 23 and 27are formed during Step 2 of FIG. 1, when patterned array 18 is formedfrom plated metal sheet 13. Package 50 is the same as package 37 of FIG.9, except for the side surfaces of the die pad and leads.

[0064] In FIG. 11, the reentrant portion of side surfaces 23 of die pad20 and side surfaces 27 of leads 24 includes a central depression 42. Inaddition, side surfaces 23 and 27 have a roughly-textured surface thatincludes numerous aspirates on the reentrant surface. Encapsulantmaterial flows into central depression 42 and into the areas of theaspirates during Step 5 of FIG. 1. The reentrant portion and theaspirates engage encapsulant material 32 and lock die pad 20 and leads24 to encapsulant material 32.

[0065] Referring to FIG. 11, reentrant side surfaces 23 of die pad 20and side surfaces 27 of leads 24 can be formed during step 2 by etchingmetal sheet 13 with a conventional liquid etchant using a controlledetch process. The etch process is continued beyond the time that wouldbe required to form orthogonal side surfaces for the die pad and leads.This is usually accomplished by using an oversized mask on upper secondsurface 15 of metal sheet 13 (FIG. 2) and using a slight overetch. Thesize and shape of depression 42 of FIG. 11 is controlled by the amountof over-etching.

[0066]FIG. 13 shows a second alternative package 60 that may be formedby the method of FIG. 1. Package 60 is identical to package 50 of FIG.11, except that the side surfaces of die pad 20 and leads 24 have adifferent reentrant profile. Referring to FIG. 13, side surfaces 23 ofdie pad 20 have a projecting lip 61 adjacent to upper second surface 22.Lip 61 includes a roughly textured surface with aspirates. Side surface23 of die pad 20 has a reentrant orthogonal portion 62 beneath lip 61,i.e., between lip 61 and lower first surface 21. Leads 24 also have asimilar projecting lip 64 with aspirates. Side surfaces 27 of leads 24also have a reentrant orthogonal portion 65 beneath lip 64, i.e.,between lip 64 and lower first surface 25. During Step 5 of the abovedescribed process, encapsulant material 32 covers lips 61 and 64, andflows beneath lips 61 and 64 to contact reentrant orthogonal portions 62and 65. The encapsulant material beneath lips 61 and 64 lock die pad 20and leads 24 to encapsulant material 32.

[0067] The reentrant side surfaces of die pad 20 and leads 24 of FIG. 13are formed during Step 2 of the process of FIG. 1. In particular, afirst step provides a patterned metal sheet having orthogonal sidesurfaces on die pad 20 and leads 24, as would be produced in a standardchemical etching process. A second step coins upper second surface 22 ofdie pad 20 and upper second surface 26 of leads 24. Coining involvesapplying a high pressure impact to upper second surfaces 22 and 26. Thishigh pressure impact deforms the side surfaces of die pad 20 and leads24 to form lips 61 and 64 (FIG. 13).

[0068]FIG. 15 is a flow chart of another method 70 within the presentinvention of forming a package. Referring to FIG. 16, Step 1 of method70 provides a plastic sheet 10. Step 2 applies a metal layer 16 to afirst surface 11 of plastic sheet 10. Metal layer 16 may be applied in avariety of ways. For example, metal layer 16 may be applied bysputtering or chemical vapor deposition. Alternatively, metal layer 16may be a metal sheet that is attached to first surface 11 using anadhesive present either on the metal sheet or on first surface 11. Forthe purpose of example, assume that metal layer 16 is copper plated withnickel, although other types of metal may be used.

[0069] Step 3 applies a photoresist pattern to the exposed secondsurface 15 of metal layer 16. The photoresist pattern is used to definea plurality of package sites 19, each including a die pad 20 andadjacent leads 24. Step 4 applies a second metal layer 17 on the exposedareas of metal layer 16. As an example, assume that second metal layer17 is gold and that the gold is plated on the nickel layer, which inturn is plated on the copper layer. Next, the photoresist pattern isstripped away, leaving gold areas where die pads 20 and leads 24 will beformed, as shown in FIG. 16.

[0070] Step 5 selectively etches the first metal layer 16 using secondmetal layer 17 as an etching mask. In this example, the chemicalselected as the etchant, typically an acid, must etch copper and nickelwithout etching or substantially etching the gold. An array of packagesites 19 is thus formed, as shown in FIG. 3.

[0071]FIG. 17 is a cross-sectional side view of a die pad 20 and twoleads 24 of a package site 19 produced by Step 5 of method 70 using theexemplary metals discussed above. Die pad 20 and leads 24 include afirst layer of copper 14, an intermediate layer of nickel 14 a and a toplayer of gold 17. The side surfaces 23 and 27 of die pad 20 and leads24, respectively, have reentrant portions in copper layer 14 and nickellayer 14 a. The reentrant portions of side surfaces 23 and 27 fill withencapsulant during a subsequent encapsulation step, and thereby lock diepad 20 and leads 24 to the encapsulant.

[0072] Steps 6-12 of method 70 are similar to Steps 3-9 of method 5 ofFIG. 1, and thus do not require further discussion. As with method 5,method 70 may be performed as a reel-to-reel process.

[0073]FIG. 18 is a flow chart of an alternative method 80 within thepresent invention of making a package. Step 1 of method 80 provides aplastic sheet 10, which may be, for example, polyimide. Alternatively,plastic sheet 10 may be polyester or polyethermide. Plastic sheet 10 mayhave an adhesive layer on first surface 11 thereof, but an adhesivelayer is not necessary. Referring to FIG. 19, Step 2 applies anelectrically conductive, metal-containing first media 81 onto firstsurface 11 of plastic sheet 10 in a pattern using a silk screeningmethod. The pattern defines an array 18 of package sites 19, each havinga die pad 20 and leads 24. Step 3 applies an optional secondelectrically conductive, metal-containing second media 82 onto firstmedia 81 using a silk screening method. In this embodiment, second media82 is applied so as to overhang all of the peripheral edges of media 81so as to form die pads 20 and leads 24 with side surfaces 23 and 27having a reentrant profile that can lock die pads 20 and leads 24 to theencapsulant. The silk screening may be performed through a 1000 meshstainless steel screen.

[0074] First media 81 and second media 82 of FIG. 19 may be lowtemperature, adhesive, metal-containing materials. For example,epoxy-based media or ink-based media may be used. The metals containedtherein may vary. For example, first media 81 and second media 82 maycontain copper and silver, respectively. Gold or aluminum containingmedia also may be used. A low temperature media is used to avoid meltingplastic sheet 10. Aluminum bond wires may be used.

[0075] In an alternative embodiment, a stencil method is used instead ofa silk screening method to form package sites 19.

[0076] Steps 4-10 of method 80 are similar to the steps 3-9 of method 5of FIG. 1, and thus do not require detailed discussion. With respect toStep 8 of method 80, plastic sheet 10 may be removed from array 18 byany of the methods discussed above. For example, plastic sheet 10 may beheated and peeled from array 17. Alternatively, array 18 may be placedin a solvent that either dissolves an adhesive connection between array18 and plastic sheet 10, or dissolves plastic sheet 10 itself. As withmethod 5 of FIG. 1, method 80 of FIG. 18 may be performed as areel-to-reel process.

[0077] The embodiments described herein are merely examples of thepresent invention. Artisans will appreciate that variations are possiblewithin the scope of the claims.

I claim:
 1. A method of forming a package for an integrated circuitdevice comprising: providing a plastic sheet having a first surface;applying a first metal layer to the first surface of the plastic sheet;patterning the first metal layer so as to form an array of packagesites, wherein each package site formed includes a die pad and aplurality of leads adjacent to the die pad; placing an integratedcircuit device on each die pad of the array; electrically connectingeach integrated circuit device to at least some of the leads adjacent tothe die pad on which the integrated circuit device is placed; applyingan encapsulating material onto the array and the first surface of theplastic sheet so that each package site is encapsulated; removing theplastic sheet from the array, and separating individual packages fromthe encapsulated array, wherein a planar first surface of the die padand leads of the packages is exposed at an external surface of eachpackage.
 2. The method of claim 1, wherein removing the plastic sheetcomprises using a solvent to dissolve a connection between the array andthe plastic sheet so that the plastic sheet may be separated from thearray.
 3. The method of claim 2, wherein an adhesive material is appliedto the first surface of the plastic sheet prior to application of themetal layer.
 4. The method of claim 1, wherein applying the first metallayer to the first surface of the plastic sheet comprises: providing asheet of metal: and attaching the sheet of metal to the first surface ofthe plastic sheet with an adhesive.
 5. The method of claim 4, whereinremoving the plastic sheet comprises using a solvent to dissolve aconnection between the array and the plastic sheet so that the plasticsheet may be separated from the array.
 6. The method of claim 1, whereinthe first metal layer comprises a layer of one type of metal plated withone or more layers of another type of metal.
 7. The method of claim 1,wherein patterning the metal layer comprises: applying a second metallayer in a pattern on the first metal layer, wherein the pattern of thesecond metal layer defines said package sites; and selectively etchingthe first metal layer to form said package sites using said second metallayer as a mask.
 8. The method of claim 7, wherein the first metal layercomprises copper and the second metal layer is gold.
 9. The method ofclaim 8, wherein the first metal layer comprises a layer of copper and alayer of nickel, wherein said gold is applied onto said layer of nickel.10. A method of forming a package for an integrated circuit devicecomprising: providing a substrate sheet having a first surface; applyinga first layer of an electrically conductive material on the substratesheet, wherein said first layer is applied in a pattern that defines anarray of package sites, wherein each package site formed includes a diepad and a plurality of leads adjacent to the die pad; placing anintegrated circuit device on each die pad of the array; electricallyconnecting each integrated circuit device to at least some of the leadsadjacent to the die pad on which the integrated circuit device isplaced; applying an encapsulating material onto the array and the firstsurface of the substrate sheet so that each package site isencapsulated; removing the substrate sheet from the array, andseparating individual packages from the encapsulated array, wherein aplanar first surface of the die pad and leads of the packages is exposedat an external surface of the package.
 11. The method of claim 10,further comprising applying a second layer of an electrically conductivematerial on the die pads and leads so that the second layer overhangsthe first layer.
 12. The method of claim 11, wherein the substrate sheetis a plastic material.
 13. The method of claim 12, wherein the first andsecond layers of electrically conductive material are applied by silkscreening.
 14. The method of claim 12, wherein the first and secondlayers of electrically conductive material are applied using a stencil.15. A method of forming a package for an integrated circuit devicecomprising: providing a plastic sheet having a first surface; applying ametal layer to the first surface of the first plastic sheet; patterningthe metal layer to form a die pad and a plurality of leads adjacent tothe die pad; placing an integrated circuit device on the die pad;consecutively connecting the integrated circuit device to one or more ofthe leads; applying an encapsulating material so that the integratedcircuit device and the first surface of the plastic sheet are covered bythe encapsulant material; and removing the plastic sheet, wherein aplanar first surface of the die pad and a first surface of the leads areexposed at an exterior surface of the package.
 16. The method of claim15, wherein removing the plastic sheet comprises using a solvent todissolve a connection between the die pad and leads and the plasticsheet so that the plastic sheet may be separated therefrom.
 17. Themethod of claim 15, wherein applying the first metal layer to the firstsurface of the plastic sheet comprises: providing a sheet of metal: andattaching the sheet of metal to the first surface of the plastic sheetwith an adhesive.
 18. The method of claim 15, wherein patterning themetal layer comprises: applying second metal layer in a pattern on thefirst metal layer, wherein the pattern of the second metal layer definessaid package sites; and selectively etching the first metal layer toform said package sites using said second metal layer as a mask.
 19. Themethod of claim 18, wherein the first metal layer comprises copper andthe second metal layer is gold.
 20. A method of forming a package for anintegrated circuit device comprising: providing a substrate sheet havinga first surface; applying a first layer of a electrically conductivematerial on the substrate sheet, wherein said first layer is applied ina pattern that defines an array of package sites, wherein each packagesite formed includes a die pad and a plurality of leads adjacent to thedie pad; placing an integrated circuit device on the die pad;consecutively connecting the integrated circuit device to one or more ofthe leads; applying an encapsulating material so that the integratedcircuit device and the first surface of the substrate sheet are coveredby the encapsulant material; and removing the substrate sheet, wherein aplanar first surface of the die pad and a first surface of the leads areexposed at an exterior surface of the package.
 21. The method of claim20, further comprising applying a second layer of an electricallyconductive material on the die pads and leads so that the second layeroverhangs the first layer.
 22. The method of claim 21, wherein thesubstrate sheet is formed of a plastic material, and the plastic sheetis removed by peeling or dissolving the plastic sheet in a solvent. 23.The method of claim 21, wherein the substrate sheet is formed of aplastic material, and an adhesive connection between the plastic sheetand the encapsulated array is dissolved with a solvent.
 24. The methodof claim 1, wherein removing the plastic sheet includes subjecting theplastic sheet to ultraviolet light.